The present invention relates to an active material, and a positive electrode and lithium-ion secondary battery using the same.
Laminar oxides (LiCoO2, LiNiO2, LiNiMnCoO2, etc.), spinel compounds (LiMn2O4, etc.), and lithium-containing phosphates (LiFePO4, etc.) have conventionally been known as positive electrode active materials for lithium-ion secondary batteries.
On the other hand, Li has been known to be reversibly inserted into and de-inserted from crystals represented by structural formulas of VOPO4 and LiVOPO4 (see Japanese Patent Application Laid-Open No. 2003-68304; N. Dupre et al., Solid State Ionics, 140, pp. 209-221 (2001); and N. Dupre et al., J. Power Sources, 97-98, pp. 532-534 (2001)). LiVOPO4 has been under study as a material substituting for the above-mentioned laminar oxide or spinel compound, since it can enhance the safety of batteries and the stability of battery characteristics. While LiVOPO4 takes a number of crystal systems, electrochemical characteristics of its orthorhombic crystal (β type) and triclinic crystal (α type) have been reported. Japanese Patent Application Laid-Open No. 2004-303527 discloses a method of improving a rate characteristic of LiVOPO4 by increasing its specific surface area with orthorhombic LiVOPO4 crystals having a median size of 20 μm or less.